Subdivision Topology : Cut Triangle into Plane
https://topologyguides.com/ is a cool site that explores all sorts of topology issues.  

First of all I'd try to align the edges of your triangle with the grid of the plane.

The rest of it depends on what the rest of your mesh needs.  If you just need a flat plane with a triangle cut out, I'd do something like this and take those extra verts straight out to the edges:



that's all quads except for the two triangles on the left and right sides.  It subdivides relatively well with just a little pinching at the corners:



if you don't want to extend all the way to the edges of the plane you can keep the edges relatively clean but close to the triangle:

:

It still subdivides ok, but a bit more pinching, and is more triangles and fewer quads:



There may be a better way, but this works for me. Definitely check out the link to the Topology Guides site, though, because it's a fantastic resource.